EPIR effect of Cu2O films by electrochemical deposition

Cuprous oxide (Cu₂O) films and Cu/Cu₂O/Cu/FTO sandwich structures were prepared by electrochemical deposition on conductive FTO substrates with different pH value conditions but constant deposition potential. The phase composition, crystal structure and microstructure of the Cu₂O films were characterized by XRD, SEM and EDS as well as by Electric–Pulse–Induced–Resistance (EPIR) perturbation. In particular, the switching effects of the Cu/Cu₂O/Cu/FTO device are examined in this work. The result shows that the EPIR-effect is large for the Cu/Cu₂O/Cu/FTO device at room temperature and strongly related to the pH value of the solution. In both acidic and neutral conditions, for example at pH = 5, 6 and 7, the EPIR effect is significant and decreases with increasing pH value. It disappears when the pH value goes further into the alkaline regime, i.e. pH = 8, 9 and 10. Space charge barriers at the interface of electrode and Cu₂O are used to explain the I–V characteristic of the layer structure and the EPIR-effect.     

Interface-engineered resistive switching in Ag/SrTiO3/Nd0.7Ca0.3MnO3/YBa2Cu3O7 heterostructures

For Ag/Nd_0.7Ca_0.3MnO₃/YBa₂Cu₃O₇ (Ag/NCMO/YBCO) heterostructures, we investigate effects of an SrTiO₃ (STO) buffer layer inserted into the Ag/NCMO interface upon the room-temperature resistive switching. In comparison with the non-buffered (Ag/NCMO/YBCO) structure, the insertion of the STO buffer layer can greatly enhance the electric-field-induced-resistance (EPIR) effect. The STO-buffered (Ag/STO/NCMO/ YBCO) device can be switched on-and-off between the higher to lower resistance states at an EPIR ratio of 253% with pulsed voltage ±1.5 V and 405% with pulsed voltage ±3.0 V. The enhanced EPIR ratio is attributed to a combined effect of the migration of oxygen vacancies near the interface and ferroelectric polarization of the STO buffer.     

The first-principle calculation of structures and defect energies in tetragonal PbTiO3

The first-principle calculation had been adopted to investigate various neutral vacancies and vacancy pairs under seven thermodynamic conditions in bulk PbTiO₃. The electronic structures, atomic relaxations, and formation energies of vacancies were obtained. Depending on the thermodynamic condition, the main and stable defects are different. It was found that V_O is the main defect under the reducing condition, whereas V_Pb becomes dominating under the oxidizing condition. The Pb-O vacancy pair forms more easily than the isolated vacancies under certain thermodynamic condition. Due to the introducing of vacancies, the acceptorlike levels and donorlike levels appear in the cases of the cation and anion vacancies, respectively.     

Resistance switching in Fe-doped P0.7Ca0.3MnO3 thin films

An enhanced electric-pulse-induced resistance (EPIR) switching effect is observed in the Ag/P_0.7Ca_0.3Mn_1 − xFe_xO₃/YBa₂Cu₃O₇ (Ag/PCMFO/YBCO) and Ag/(PCMFO/PCMO)/YBCO structures. Unlike in the PCMO-based EPIR devices, where the Ag/PCMO interface plays a crucial role, both the Ag/PCMFO interface and the bulk PCMFO are found to have significant contributions to the EPIR switching of the PCMFO-based device. A possible explanation is to extend the pulse-driven oxygen ion/vacancy motion model at the metal/PCMO interface region to the bulk PCMFO.     

Photoluminescence of hexagonal boron nitride: Effect of surface oxidation under UV-laser irradiation

We report on the UV laser-induced fluorescence of hexagonal boron nitride (h-BN) following nanosecond laser irradiation under vacuum and in different environments of nitrogen gas and ambient air. The observed fluorescence bands are tentatively ascribed to impurity and mono (V_N) or multiple (m-V_N with m=2 or 3) nitrogen vacancies. A structured fluorescence band between 300 and 350 nm is assigned to impurity-band transition and its complex lineshape is attributed to phonon replicas. An additional band at 340 nm, assigned to V_N vacancies on surface, is observed under vacuum and quenched by adsorbed molecular oxygen. UV-irradiation of h-BN under vacuum results in a broad asymmetric fluorescence at ∼400 nm assigned to m-V_N vacancies; further irradiation breaks more B-N bonds enriching the surface with elemental boron. However, no boron deposit appears under irradiation of samples in ambient atmosphere. This effect is explained by oxygen healing of radiation-induced surface defects. Formation of the oxide layer prevents B-N dissociation and preserves the bulk sample stoichiometry.     

Mixed conducting perovskite-like ceramics on the base of lanthanum gallate

Ceramic perovskite solid solutions (La_0.9Sr_0.1)[(Ga_1−xM_x)_0.8Mg_0.2]O_3−y, 0 ≤ x ≤ 0.5, M = Fe, Ni, Cr (systems I–III) and brownmillerite solid solutions (La_0.2Sr_1.8)[Ga(Fe_1−xMg_x)]O_5−z, 0 ≤ x ≤ 0.5, (system IV) have been prepared. The samples have been studied by X-ray diffraction and electron microscopy methods, dielectric spectroscopy and permeability measurements. The correlation between the composition, unit cell parameter changes, electrical transport and oxygen permeation properties has been revealed. Introduction of transition metals (Fe, Ni, or Cr), substituting for gallium, ensures the enhancement of the electronic constituent of the conductivity in the perovskite systems I–III. Stabilization of the transition metal high valence states 4+ or 5+ has been suggested for compositions I and III. This leads to a unit cell volume contraction and provides a decrease in the concentration of oxygen vacancies. The oxygen permeability reaches its maximum values in compositions I–III with x ∼ 0.3. On the contrary, increasing concentration of the doping element with lower valence state (magnesium), substituting for iron, determines the expansion of the brownmillerite unit cell volume and provides an increase of the oxygen vacancy concentration, which in turn, favors the enhancement of oxygen permeability of composition IV.     

点接触金属/Pr$lt;sub$gt;0.7$lt;/sub$gt;Ca$lt;sub$gt;0.3$lt;/sub$gt;MnO$lt;sub$gt;3$lt;/sub$gt;/Pt结构稳定的低电流电阻开关特性

利用自主开发的导电原子力显微镜控制Pt,W探针构成点接触金属/Pr_0.7Ca_0.3MnO₃（PCMO）/Pt三明治结构,对其电流-电压（I-V）及脉冲诱导电阻开关（EPIR）特性进行了研究.研究发现,在10 nA限流下两种电极对应结构的I-V都表现出相当稳定的双极性电阻开关特性,以及大于100的电阻开关比.进一步测试发现,点接触W/PCMO/Pt器件具有在10 nA限流下稳定的EPIR特性以及100 pA限流下重复的双极性电阻开关特性.此电流比已报道的电流低3个数量级,表明此结构在低功耗存储器件方面的潜在应用.通过对比样品不同位置、不同限流、不同接触面积点接触Pt/PCMO/Pt的I-V回滞特性,把点接触器件在低电流下稳定、显著的电阻开关效应归结于小的器件面积导致强的局域电场加强了O离子迁移效应. 关键词： 脉冲诱导电阻开关 电场下氧离子迁移 电阻开关     

Electric-pulse-induced resistance switching in Nd0.7Sr0.3MnO3 ceramics

Nonlinear and hysteretic current–voltage characteristics and electric-pulse-induced resistance (EPIR) switching effects were found in Nd_0.7Sr_0.3MnO₃ ceramics. For samples measured using the two-wire method, a polarity-dependent, stable and reproducible EPIR effect occurs in the whole temperature range measured. The high and low resistive metastates vary with temperature and seemingly can be destroyed by temperatures larger than 400 K. However, for the same sample measured using the four-wire method, no EPIR effect is observed. The result indicates that the EPIR effect is intensively correlated with the interface between the Ag-electrodes and the bulk of the Nd_0.7Sr_0.3MnO₃ polycrystal.     

Study of anti-clockwise bipolar resistive switching in Ag/NiO/ITO heterojunction assembly

The anti-clockwise bipolar resistive switching in Ag/NiO/ITO (Indium–Tin–Oxide) heterojunctional thin film assembly is investigated. A sequential voltage sweep in 0 → V _max → 0 → ?V _min → 0 order shows intrinsic hysteresis behaviour and resistive switching in current density (J)–voltage (V) measurements at room temperature. Switching is induced by possible rupture and recovery of the conducting filaments in NiO layer mediated by oxygen ion migration and interfacial effects at NiO/ITO junction. In the high-resistance OFF-state space charge limited current passes through the filamentary path created by oxygen ion vacancies. In OFF-state, the resistive switching behaviour is attributed to trapping and detrapping processes in shallow trap states mostly consisting of oxygen vacancies. The slope of Log I vs Log V plots, in shallow trap region of space charge limited conduction is ~2 (I ∝ V ²) followed by trap-filled and trap-free conduction. In the low-resistance ON-state, the observed electrical features are governed by the ohmic conduction.     

Diffusion of oxygen vacancies in yttrium iron garnet investigated by dynamic conductivity measurements

In yttrium iron garnet, Y₃Fe₅O₁₂, the oxygen vacancy concentration at high temperatures depends on the partial oxygen pressure. Due to the electron donating nature of the vacancies, changes in the oxygen vacancy concentration can be measured by electrical conductivity measurements. We discuss a dynamic method for studying the diffusion of oxygen vacancies by measurements of the time dependence of the electrical conduction after a change in the oxygen partial pressure has taken place. It is shown that the interpretation of the measurements is straightforward if the relative change in conductivity remains small (? 10%). Measurements were performed on single crystals and on polycrystalline samples at temperatures 900–1400°C. The samples were made n-type by substitution with Si or p-type by substitution with Ca, Zn or Pb. The partial oxygen pressure was changed between 1 and 0.1 atm. For all samples the diffusion coefficient D of the oxygen vacancies can be represented by $\text{D = A}\text{exp}\text{(?}\text{Q}\text{kT}\text{)}$, where A = 8400 cm²s^?1 and Q = 2.90 eV. It is shown that the activation energy of 2.90 eV is due to the migration enthalpy of the vacancies only.     

Low temperature electronic transport in sputter deposited a-IGZO films

We report on the electrical properties of a-IGZO thin films prepared by reactive sputtering. Without oxygen injection, dc resistivity measured at room temperature is ρ_300K = 1.22 × 10⁻³ Ωm. The lowest resistivity ρ_300K = 4.86 × 10⁻⁵ Ωm is obtained at a certain oxygen supply into the deposition process. Hall effect measurements of these films reveal a metallic-like behavior from mobility and carrier concentration vs. temperature in the range 15–300 K whereas films deposited without oxygen or for the highest oxygen flows behave as semiconductors. These enhanced electrical properties are connected to the oxygen vacancies and the local coordination structure around the In³⁺ cations.     

Photovoltaic characteristics of Au/PVA (Bi-doped)/n-Si Schottky barrier diodes (SBDs) at various temperatures

The charge conduction properties of the Au/PVA (Bi-doped)/n-Si Schottky barrier diodes (SBDs) were investigated using current–voltage–temperature (I–V–T) measurements in dark and under various illumination levels. For this purpose, the main diode parameters such as reverse-saturation current (I_o), zero-bias barrier height (Φ_Bo), ideality factor (n), series resistance (R_s) and shunt resistance (R_sh) of diode were obtained as function of temperature and illumination level. Experimental results show that all of these electrical parameters are strong functions of illumination and temperature. The change in all electrical parameters becomes more important at low temperatures and illumination levels. While the n value decreases with increasing temperature and illumination level, Φ_Bo value increases. The fill factor (FF = V_m·I_m/V_oc·I_sc) values were obtained as 0.34 at 80 K and 0.40 at 320 K under 50 W and these values are near to a photodiode. Therefore, the fabricated diode can be used as a photodiode in optoelectronic applications. The forward bias I–V characteristics of the diode have also been explained by the space charge limited current (SCLC) model.     

Effects of copper and oxygen vacancies on the ferromagnetism of Mn- and Co-doped Cu2O

The ferromagnetic properties of Mn- and Co-doped Cu₂O with copper and oxygen vacancies (V_Cu and V_O) are studied by first-principles calculations. The results indicate that Mn-doped Cu₂O has an antiferromagnetic state in the near configuration, while it has a ferromagnetic state for the far configuration. On the contrary, Co-doped Cu₂O possess a ferromagnetic state regardless of the distance between the two Co atoms. The observed ferromagnetism can be attributed to the 90° ferromagnetic super-exchange mechanism. The presence of V_O can enhance the ferromagnetism, whereas V_Cudepresses it.     

Resistive switching functional quantum-dot light-emitting diodes

This study demonstrates quantum-dot light-emitting diodes (QD-LEDs) with a function of resistive switching memory, capable of on/off operation at the same driving current depending on reset/set state. The QD-LEDs were fabricated by spin-coating process and experienced two different annealing conditions, which yielded defective or less-defective V₂O_5–x layer. One of the annealing conditions produced QD-LEDs with the unusual electrical behaviors of negative differential resistance (NDR), capacitance oscillation, and voltage–current hysteresis curves, signifying so-called resistive switching characteristics. X-ray and ultraviolet photoelectron spectroscopies were used to examine the chemical state of the differently annealed V₂O_5–x layers. The less stoichiometric V₂O_5–x layer was found to be responsible for the resistive switching behaviors of the NDR and the low and high resistance states (LRS and HRS, respectively). We discuss the LRS/HRS of V₂O_5–x for resistive switching in terms of a conductive filament effect, induced by microstructural changes caused by oxygen drift and vacancy annihilation processes in the high defect density V₂O_5–x layer.     

A close correlation between induced ferromagnetism and oxygen deficiency in Fe doped In2O3

We report on the reversible manipulation of room temperature ferromagnetism in Fe (5%) doped In₂O₃ polycrystalline magnetic semiconductor. The X-ray diffraction and photoemission measurements confirm that the Fe ions are well incorporated into the lattice, substituting the In³⁺ ions. The magnetization measurements show that the host In₂O₃ has a diamagnetic ground state, while it shows weak ferromagnetism at 300 K upon Fe doping. The as-prepared sample was then sequentially annealed in hydrogen, air, vacuum and finally in air. The ferromagnetic signal shoots up by hydrogenation as well as vacuum annealing and bounces back upon re-annealing the samples in air. The sequence of ferromagnetism shows a close inter-relationship with the behavior of oxygen vacancies (V_o). The Fe ions tend to a transform from 3+ to 2+ state during the giant ferromagnetic induction, as revealed by photoemission spectroscopy. A careful characterization of the structure, purity, magnetic, and transport properties confirms that the ferromagnetism is due to neither impurities nor clusters but directly related to the oxygen vacancies. The ferromagnetism can be reversibly controlled by these vacancies while a parallel variation of carrier concentration, as revealed by resistance measurements, appears to be a side effect of the oxygen vacancy variation.     

Effect of Zn substitution for Co on the transport properties of Y BaCo4O7

Electrical resistivity and Seebeck coefficients of Y BaCo_4−xZn_xO₇ (x=0.0,0.5,1.0,2.0) were investigated in the temperature range 350-1000 K. It was found that the electrical resistivity and activation energy increase with increasing Zn concentration, while Seebeck coefficients do not increase but decrease when electrical resistivity increases. We explained the increase of electrical resistivity and the drop of Seebeck coefficients for Zn-substituted samples by the decrease of carrier mobility, rather than of carrier concentration. The effect of oxygen absorption and desorption on the electrical resistivity and Seebeck coefficients was also investigated. An abrupt change of transport properties happens at about 650 K for x=0.0 and 0.5 samples measured in oxygen. For x=1.0 and 2.0 samples, however, such change disappears and the transport behavior in oxygen is almost same as that in nitrogen due to the significant suppression of oxygen diffusion caused by the higher Zn concentration in these samples.     

Mesure de la conductivite electrique totale du protoxyde de cobalt en fonction de sa composition definie par des electrodes d'alliage cobalt-platine,entre 1000 et 1400 K

The total electrical conductivity of cobaltous oxide has been measured from 1000 to 1400 K as a function of its composition, especially in the range of low oxygen pressures up to the limit of equilibrium with cobalt. These measurements have been carried out with monocrystalline samples of oxide placed between cobalt-platinum alloy electrodes; oxide composition is fixed by the activity of cobalt in the alloy which is also the activity of cobalt in oxide. Results are given by isotherm curves of the logarithm of conductivity versus the logarithm of oxygen pressure. The slope of these isotherms shows a regular variation between $\text{1}\text{4}$ and less than $\text{1}\text{6}$; this fact can only be explained by the successive effects of the formation of vacancies V″_Co, V′_Co and of intrinsic conductivity by electrons and free holes.     

La0.5Ca0.5MnO3内禀与界面电脉冲诱导电阻转变效应的比较

一般地,钛矿结构锰氧化物的电脉冲诱导电阻转变(EPIR) 效应源于非内禀界面处的肖特基势垒. 本文采用固相烧结法制备了La_0.5Ca_0.5MnO₃ (LCMO)陶瓷样品, 用四线测量模式对样品电输运性质, 特别对其内禀EPIR效应和忆阻器行为进行了研究. 室温下, 尽管样品在四线测量模式下的I-V特性曲线呈欧姆线性规律, 但在适当的脉冲电压刺激下, 仍能诱导产生明显、稳定的EPIR效应. 通过与二线模式的界面EPIR比较, 发现LCMO内禀EPIR效应具有更小的脉冲临界电压、更好的稳定性和抗疲劳特性, 是稀土掺杂锰氧化物中观察到的一类新颖的EPIR效应. 关键词： 钙钛矿结构锰氧化物 电致电阻效应 电脉冲诱导电阻转变效应 肖特基势垒     

Enhancement of the electrical characteristics of indium–zinc tin-oxide thin-film transistors utilizing dual-channel layers

Thin film transistors (TFTs) with indium–zinc tin-oxide (IZTO) dual-channel layers were fabricated on heavily-doped p-type Si substrates by using a tilted dual-target radio-frequency magnetron sputtering system. The number of oxygen vacancies in the IZTO channel layer decreased with increasing oxygen partial pressure, resulting in a decrease in the conductivity. The threshold voltage (V_th) shifted toward positive gate-source voltage with increasing oxygen partial pressure for the growth of the IZTO layer because of a decrease in the carrier concentration. The V_th, the mobility, the on/off-current ratio, and the subthreshold swing of the dual-channel IZTO TFTs were 3.5 V, 7.1 cm²/V s, 1.3 V/decade, and 8.2 × 10⁶, respectively, which was enhanced by utilizing dual-channel layers consisting of a top channel deposited at a high oxygen partial pressure and a bottom channel deposited at a low oxygen partial pressure.     

Neutron diffraction analysis of a defect vanadium monoxide close to the equiatomic vanadium monoxide

Neutron and X-ray diffraction analyses are applied to studying the defect structure of synthesis-temperature quenched and low-temperature annealed vanadium monoxides VO _y (0.90 ≤ y ≤ 0.97) close to the equiatomic monoxide VO_1.0. It is found that the monoxides VO_0.90 and VO_0.97 contain structural vacancies not only in the oxygen sublattice, but also in the metal sublattice. In addition to the cubic disordered phase VO _y with the structure B1, the monoclinic superstructure V₁₄O₆ with space group C2/m is present in the synthesized VO_0.90 sample and in the annealed VO_0.90 and VO_0.97 samples. The formation of the V₁₄O₆ superstructure is attributed to the ordering of oxygen atoms and nonmetal vacancies in the lattice of the tetragonal solid solution of oxygen in vanadium. No simultaneous ordering of metal and oxygen vacancies in two sublattices of the cubic vanadium is observed.